How to zoom in and out the user presp
To see all objects in the scene:


in the 3D Viewport footer: : View -> View all
or press Home


If you have a mouse with a mouse wheel: 


Scroll with the wheel.
Or press Ctrl MMB  then as long as you keep the the key and the MMB  pressed, you can zoom in/out by dragging the mouse.


If you have a 2 button mouse:


Open user preferences
Open the Input Tab
enable Emulate 3 Button Mouse
then you can press Ctrl Alt then click LMB  and drag the mouse to zoom in/out